This application claims the benefit of Japanese Patent Applications No.11-293717 filed Oct. 15, 1999 and No. 2000-090475 filed Mar. 29, 2000, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to pulse modulation circuits, optical write units, image forming apparatuses and optical write methods, and more particularly to a pulse width modulation circuit which is used to control an optical output of a light source in a laser printer, a LED printer, an optical disk unit, a digital copying machine, an optical communication unit and the like, and to an optical write unit which uses a modulated signal output from such a pulse width modulation circuit. The present invention further relates to an image forming apparatus, such as a laser printer, a LED printer, a digital copying machine and a facsimile machine, which uses such an optical write unit, and to an optical write method which carries out an optical write operation by employing a pulse width modulation technique.
2. Description of the Related Art
As techniques for modulating an optical output of a light source, there are the power modulation technique which modulates the light quantity itself, the pulse width modulation technique which modulates the ON-time of the light source, and the power-pulse width mixed modulation technique which combines the power modulation technique and the pulse width modulation technique. According to a first pulse width modulation technique, a triangular wave or a sawtooth wave is generated at a corresponding pulse generation period, and is compared with an analog video signal in a comparator so as to generate a pulse width modulated signal. On the other hand, according to a second pulse width modulation technique, a high-frequency clock is digitally frequency-divided so as to generate a delayed pulse, and a pulse width modulated signal is generated by obtaining a logical sum or a logical product of the high-frequency clock and the delayed pulse.
For example, a Japanese Laid-Open Patent Application No.6-284276 proposes an image forming apparatus which makes a gradation representation of an image by the pulse width modulation. This proposed image forming apparatus is provided with a means for determining a delay quantity from an input image signal based on a predetermined reference clock, and a means for generating a signal having a predetermined pulse width which is delayed by a delay time set by the determined delay quantity, and the pulse width modulation is carried out according to the signal having the predetermined pulse width.
On the other hand, a Japanese Laid-Open Patent Application No.9-183250 proposes an image forming apparatus which is provided with a scanning means for scanning a rotary photoconductive body by a light from a light source modulated based on an image signal, a scanning light detecting means for detecting a scanning light from the scanning means at a predetermined position with respect to the rotary photoconductive body, and a reference oscillator. An electrostatic latent image which is dependent on the image signal is formed by scanning the rotary photoconductive body at a predetermined timing based on a detection signal from the scanning light detecting means and an oscillation output of the reference oscillator, and an image dependent on the electrostatic latent image is formed on a recording medium. This proposed image forming apparatus is further provided with a high-frequency clock generating means for generating a high-frequency clock having a frequency n times the oscillation output of the reference oscillator, where n is an integer satisfying nxe2x89xa72, an image processing clock generating means for generating an image processing clock signal which is synchronized to the detection signal from the scanning light detecting means based on the high-frequency clock and the detection signal, and a modulating means for modulating the image signal based on the high-frequency clock and generating an image modulated signal, so as to drive the light source based on the image modulated signal.
In image forming apparatuses, there are demands to increase the operation speed. However, in the image forming apparatus which employs the first pulse width modulation technique, it is impossible to realize both the high operation speed and the linearity and reproducibility of the triangular or sawtooth wave. In addition, in the image forming apparatus which employs the second pulse width modulation technique, a maximum operation frequency depends on a device used, and it is impossible to realize both the high operation speed and the high gradation representation of the image.
For example, if a pixel clock has a frequency of 50 MHz and the pulse width modulation is to be carried out using 256 values, it is difficult to obtain satisfactory linearity and swing of the triangular or sawtooth wave at a period of 20 ns of the triangular or sawtooth wave in the case of the first pulse width modulation technique. Furthermore, in the case of the second pulse width modulation technique which digitally frequency-divides the high-frequency clock, it is difficult to realize a circuit structure which generates a clock having a frequency of 12.8 GHZ (=50 MHzxc3x97256).
Accordingly, it is a general object of the present invention to provide a novel and useful pulse modulation circuit, optical write unit, image forming apparatus and optical write method, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a pulse modulation circuit, optical write unit, image forming apparatus and optical write method, which can cope with a high operation speed, and realize a high gradation representation of an image, compact size, low cost and low power consumption.
Still another object of the present invention is to provide a pulse width modulation circuit comprising signal generating means for generating a reference clock signal and a predetermined signal which is approximately inversely proportional to a digital data input signal; delay quantity generating means for delaying the reference clock signal by a desired phase delay to output a pulse signal, based on the predetermined signal from the signal generating means; delay quantity control means for controlling a delay quantity of the delay quantity generating means; and modulated signal generating means for generating a modulated signal which is pulse-width-modulated based on the pulse signal from the delay quantity generating means and the reference clock signal. According to the pulse width modulation circuit of the present invention, it is possible to cope with a case where the operation speed is high by use of a high-speed analog delay circuit, so that a high gradation representation of an image is possible, for example.
In the pulse width modulation circuit, the signal generating means may comprises a digital-to-analog converter circuit converting the digital data input signal into an analog signal; and a division circuit generating the predetermined signal based on the analog signal from the digital-to-analog converter circuit. In this case, a high-speed pulse width generation is possible. Hence, it is possible to cope with a case where the operation speed is high by use of a high-speed analog delay circuit, so that a high gradation representation of an image is possible, for example.
In the pulse width modulation circuit, the modulated signal generating means may set a reference time width of pulse width modulation to a value less than a period of the reference clock signal. In this case, it is possible to realize a high resolution without deteriorating the gradation representation of an image.
In the pulse width modulation circuit, the modulated signal generating means may switch a reference time width of pulse width modulation between a value less than a period of the reference clock signal and a value equal to the period of the reference clock signal. In this case, it is possible to realize a high resolution without deteriorating the gradation representation of an image depending on the modulation target.
In the pulse width modulation circuit, a plurality of systems may be provided with respect to the modulated signal generating means, and each of the systems may include the signal generating means and the delay quantity generating means. In this case, it is possible to cope with even higher operation speeds.
In the pulse width modulation circuit, the digital data input signal may be made up of image data, and a writing phase within one dot may be controlled with reference to a non-inverted or inverted clock signals of the reference clock signal. In this case, it is possible to vary the density of the pulse width modulation, and improve the gradation representation at a highlighted part of the image, for example.
A further object of the present invention is to provide a pulse width modulation circuit comprising first signal generating means for generating a high-frequency clock signal synchronized to a reference clock signal by frequency-multiplying the reference signal, and generating a plurality of pulse signals having mutually different phases from the high-frequency clock signal; pulse selecting means for selecting one of the plurality of pulse signals based on upper bit data of a digital data input signal which is synchronized to the reference clock signal; second signal generating means for generating a predetermined signal which is approximately inversely proportional to lower bit data of the digital data input signal; delay quantity generating means for delaying the one of the plurality of pulse signals from the pulse selecting means by a desired phase delay to output a pulse signal, based on the predetermined signal; delay quantity control means for controlling a delay quantity of the delay quantity generating means; and modulated signal generating means for generating a modulated signal which is pulse-width-modulated based on the pulse signal from the delay quantity generating means and the one of the plurality of pulse signals. According to the pulse width modulation circuit of the present invention, it is possible to cope with a case where the operation speed is high by use of a high-speed analog delay circuit, so that a high gradation representation of an image is possible, for example.
In the pulse width modulation circuit, the second signal generating means may comprise a digital-to-analog converter circuit converting the digital data input signal into an analog signal; and a division circuit generating the predetermined signal based on the analog signal from the digital-to-analog converter circuit. In this case, a high-speed pulse width generation is possible. Hence, it is possible to cope with a case where the operation speed is high by use of a high-speed analog delay circuit, so that a high gradation representation of an image is possible, for example.
In the pulse width modulation circuit, the modulated signal generating means may set a reference time width of pulse width modulation to a value less than a period of the reference clock signal. In this case, it is possible to realize a high resolution without deteriorating the gradation representation of an image.
In the pulse width modulation circuit, the modulated signal generating means may switch a reference time width of pulse width modulation between a value less than a period of the reference clock signal and a value equal to the period of the reference clock signal. In this case, it is possible to realize a high resolution without deteriorating the gradation representation of an image depending on the modulation target.
In the pulse width modulation circuit, a plurality of systems may be provided with respect to the modulated signal generating means, and each of the systems may include the second signal generating means and the delay quantity generating means. In this case, it is possible to cope with even higher operation speeds.
In the pulse width modulation circuit, the digital data input signal may be made up of image data, and a writing phase within one dot may be controlled with reference to a non-inverted or inverted clock signals of the reference clock signal. In this case, it is possible to vary the density of the pulse width modulation, and improve the gradation representation at a highlighted part of the image, for example.
Another object of the present invention is to provide an optical write unit which drives a light source by an image modulated signal which is pulse-width-modulated based on input digital multi-level image data, and forms an electrostatic latent image by scanning a photoconductor body by scanning means using a scanning light emitted from the light source, the optical write unit comprising a pulse width modulation circuit outputting the image modulated signal; and a light source driver driving the light source by the image modulated signal output from the pulse width modulation circuit, wherein the pulse width modulation circuit comprises a signal generating circuit generating a reference clock signal and a predetermined signal which is approximately inversely proportional to the input digital multi-level image data; a delay quantity generating circuit delaying the reference clock signal by a desired phase delay to output a pulse signal, based on the predetermined signal from the signal generating circuit; a delay quantity controller controlling a delay quantity of the delay quantity generating circuit; and a modulated signal generator generating the image modulated signal which is pulse-width-modulated based on the pulse signal from the delay quantity generating circuit and the reference clock signal. According to the optical write unit of the present invention, it is possible to realize a high gradation representation of an image even when the operation speed is high.
The optical write unit may further comprise a sensor detecting the scanning light from the scanning means at a predetermined position and outputting a detection signal; and a clock generator generating the reference clock signal in synchronism with the detection signal, where the clock generator, the light source driver and the pulse width modulation circuit are integrated within a single integrated circuit chip. In this case, the operation is stable with respect to electromagnetic interference (EMI), and the size, cost and power consumption of the optical write unit can all be reduced.
The optical write unit may further comprise character region recognizing means for recognizing a character region from the input digital multi-level image data and outputting a character region signal; and a write data generator generating write data based on the character region signal, wherein the pulse width modulation circuit carries out a pulse width modulation with respect to a non-character region using one period of the reference clock signal as corresponding to one pixel, and carries out a pulse width modulation with respect to the character region using one-half period of the reference clock signal as corresponding to one pixel, in response to the character region signal. In this case, it is possible to obtain a sharp image in the character region, and a smooth image having a high fidelity with respect to the original image in the non-character region of the image.
Still another object of the present invention is to provide an image forming apparatus comprising an optical write unit outputting an image modulated signal which is pulse-width-modulated based on input digital multi-level image data; a light source driven by the image modulated signal; and means for forming an electrostatic latent image by scanning a photoconductor body by scanning means using a scanning light emitted from the light source, the optical write unit comprising a pulse width modulation circuit outputting an image modulated signal; and a light source driver driving the light source by the image modulated signal output from the pulse width modulation circuit, wherein the pulse width modulation circuit comprises a signal generating circuit generating a reference clock signal and a predetermined signal which is approximately inversely proportional to the input digital multi-level image data; a delay quantity generating circuit delaying the reference clock signal by a desired phase delay to output a pulse signal, based on the predetermined signal from the signal generating circuit; a delay quantity controller controlling a delay quantity of the delay quantity generating circuit; and a modulated signal generator generating the image modulated signal which is pulse-width-modulated based on the pulse signal from the delay quantity generating circuit and the reference clock signal. According to the image forming apparatus of the present invention, it is possible to realize a high gradation representation even when the operation speed is high.
Another object of the present invention is to provide an optical write unit which drives a light source by an image modulated signal which is pulse-width-modulated based on input digital multi-level image data, and forms an electrostatic latent image by scanning a photoconductor body by scanning means using a scanning light emitted from the light source, the optical write unit comprising a pulse width modulation circuit outputting the image modulated signal; and a light source driver driving the light source by the image modulated signal output from the pulse width modulation circuit, wherein the pulse width modulation circuit comprises a first signal generating circuit generating a high-frequency clock signal synchronized to a reference clock signal by frequency-multiplying the reference signal, and generating a plurality of pulse signals having mutually different phases from the high-frequency clock signal; a selector selecting one of the plurality of pulse signals based on upper bit data of the input digital multi-level image data which is synchronized to the reference clock signal; a second signal generating circuit generating a predetermined signal which is approximately inversely proportional to lower bit data of the input digital multi-level image data; a delay quantity generating circuit delaying the one of the plurality of pulse signals from the selector by a desired phase delay to output a pulse signal, based on the predetermined signal; a delay quantity controller controlling a delay quantity of the delay quantity generating circuit; and a modulated signal generator generating the image modulated signal which is pulse-width-modulated based on the pulse signal from the delay quantity generating circuit and the one of the plurality of pulse signals. According to the optical write unit of the present invention, it is possible to realize a high gradation representation even when the operation speed is high.
Still another object of the present invention is to provide an image forming apparatus comprising an optical write unit outputting an image modulated signal which is pulse-width-modulated based on input digital multi-level image data; a light source driven by the image modulated signal; and means for forming an electrostatic latent image by scanning a photoconductor body by scanning means using a scanning light emitted from the light source, the optical write unit comprising a pulse width modulation circuit outputting the image modulated signal; and a light source driver driving the light source by the image modulated signal output from the pulse width modulation circuit, wherein the pulse width modulation circuit comprises a first signal generating circuit generating a high-frequency clock signal synchronized to a reference clock signal by frequency-multiplying the reference signal, and generating a plurality of pulse signals having mutually different phases from the high-frequency clock signal; a selector selecting one of the plurality of pulse signals based on upper bit data of the input digital multi-level image data which is synchronized to the reference clock signal; a second signal generating circuit generating a predetermined signal which is approximately inversely proportional to lower bit data of the input digital multi-level image data; a delay quantity generating circuit delaying the one of the plurality of pulse signals from the selector by a desired phase delay to output a pulse signal, based on the predetermined signal; a delay quantity controller controlling a delay quantity of the delay quantity generating circuit; and a modulated signal generator generating the image modulated signal which is pulse-width-modulated based on the pulse signal from the delay quantity generating circuit and the one of the plurality of pulse signals. According to the image forming apparatus of the present invention, it is possible to realize a high gradation representation even when the operation speed is high.
A further object of the present invention is to provide an optical write method for an image forming apparatus which drives a light source by an image modulated signal which is pulse-width-modulated based on input digital multi-level image data, and forms an electrostatic latent image by scanning a photoconductor body by scanning means using a scanning light emitted from the light source, the optical write method comprising the steps of (a) outputting the image modulated signal; and (b) driving the light source by the image modulated signal obtained by the step (a), wherein the step (a) includes (a1) generating a reference clock signal and a predetermined signal which is approximately inversely proportional to the input digital multi-level image data; (a2) delaying the reference clock signal by a desired phase delay to output a pulse signal, based on the predetermined signal; (a3) controlling a delay quantity used in the step (a2); and (a4) generating the image modulated signal which is pulse-width-modulated based on the pulse signal obtained in the step (a2) and the reference clock signal. According to the optical write method of the present invention, it is possible to realize a high gradation representation even when the operation speed is high.
Another object of the present invention is to provide an optical write unit method for an image forming apparatus which drives a light source by an image modulated signal which is pulse-width-modulated based on input digital multi-level image data, and forms an electrostatic latent image by scanning a photoconductor body by scanning means using a scanning light emitted from the light source, the optical write method comprising the steps of (a) outputting the image modulated signal; and (b) driving the light source by the image modulated signal obtained by the step (a), wherein the step (a) includes (a1) generating a high-frequency clock signal synchronized to a reference clock signal by frequency-multiplying the reference signal, and generating a plurality of pulse signals having mutually different phases from the high-frequency clock signal; (a2) selecting one of the plurality of pulse signals based on upper bit data of the input digital multi-level image data which is synchronized to the reference clock signal; (a3) generating a predetermined signal which is approximately inversely proportional to lower bit data of the input digital multi-level image data; (a4) delaying the one of the plurality of pulse signals obtained by the step (a3) by a desired phase delay to output a pulse signal, based on the predetermined signal; (a5) controlling a delay quantity used in the step (a4); and (a6) generating the image modulated signal which is pulse-width-modulated based on the pulse signal obtained by the step (a4) and the one of the plurality of pulse signals. According to the optical write method of the present invention, it is possible to realize a high gradation representation even when the operation speed is high.